More precisely, but not exclusively, the invention relates to such a system as applied to a prepayment indoor telephone set where payment is performed by means of cards, i.e. it relates to a telephone set installed on private premises, or even in a public place, where telephone charges are paid at least mainly by means of memory cards and without there being any call billing, or even without there being any customer to bill. The invention is particularly advantageous when telephone charging is performed by counting charging pulses transmitted from the telephone exchange, with user prepayment then taking place immediately. It will be understood that under such circumstances it must be impossible for the prepayment telephone set to be replaced by a normal telephone set since that would enable telephone calls to be obtained free of charge by a fraudulent user.
More generally, the invention applies to any case where mutual authentication needs to be established between a terminal (in particular a telephone) and a transmission line (in material form or otherwise) prior to authorizing data interchange.
One solution that has already been proposed for solving this problem is as follows: during the initial stage in which a telephone call is set up, coded magnitudes are interchanged between the transmitter and the receiver for the purpose of verifying that the call is authorized. Such a system solves the problem in part, only. Proposals have also been made to encode the information to be transmitted at the transmitting end to decode it symmetrically at the receiving end. However when information is transmitted in the form of analog frequency signals coding and decoding operations become relatively complex.
In order to remedy this drawback, an object of the invention is to provide such a telephone call system which enables call authorization to be verified throughout the duration of a call and which is also compatible with the transmission of information in the form of analog frequency signals, and which finally is capable of countering attempts at fraud, including recording the signals interchanged while setting up earlier calls.